Spin-polarized Atomic Hydrogen Gas Research Articles

Thermal Compression of Atomic Hydrogen on Helium Surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid 4He surface. The surface gas density is increased locally by thermal compression up to 5.5 × 1012 cm−2 at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.

Three-body recombination in two-dimensional atomic hydrogen gas

We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from $45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at $4.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.

Collisional frequency shifts of absorption lines in an atomic hydrogen gas

We consider the effect of interactions on the line shape of the two-photon 1s-2s transition in a (doubly) spin-polarized atomic hydrogen gas in terms of the interatomic interaction potentials. We show that the frequency-weighted sum rule for the intensity of the line is not given simply in terms of the pseudopotentials that describe the interactions between low-energy atoms. The origin of the departures from the simple pseudopotential result for the frequency-weighted sum rule is traced to what we refer to as incoherent contributions to the spectral weight. These arise from more complicated final states of the many-body system than the ones usually considered. In particular, we show how the relevant response function may be treated in a manner similar to the density-density response function for Fermi liquids, and express it as a coherent part coming from single particle-hole pairs, and an incoherent part coming from other excitations. We argue that in experiments only the coherent part of the response of the system is observed, and its contribution to the frequency-weighted sum rule is shown to be given correctly by the pseudopotential approximation. Finally we calculate the width of the coherent part of the line due to collisional damping.

One-dimensional evaporative cooling of magnetically trapped atomic hydrogen

We report experiments on evaporative cooling of spin-polarized atomic hydrogen gas confined in a cryogenic magnetic trap, and present a model of the trapped gas based on a generalized truncated Boltzmann approximation for the phase-space distribution function. The model takes into account the dimension of the evaporation [three-dimensional (3D) or 1D] and the time dependence of both the depth and the shape of the confining potential. Our observations are consistent with 1D evaporative cooling. To attain maximal phase-space density, we used the model assuming 1D evaporative cooling to optimize the evaporation procedure. With this work, the low dimension of evaporation was identified as the bottleneck thus far preventing us from achieving Bose-Einstein condensation in trapped atomic hydrogen.

Elementary excitations of atomic hydrogen gas on liquid helium

We discuss the nature of the elementary excitations for a gas of spin-polarized atomic hydrogen adsorbed on the free surface of liquid helium. Below the Kosterlitz-Thouless transition temperature the coupling between above-condensate hydrogen atoms and ripplons through the hydrogen quasicondensate results in the elementary excitations being superpositions of hydrogen quasiparticles (single atom at large momenta and phonon at small momenta) and ripplons. For realistic values of the condensate density this hybridization is appreciable.

Thermal detection of ESR on spin-polarized hydrogen: Study of surface recombination

We introduce a new method for the detection of electron-spin resonance in spin-polarized atomic hydrogen gas (H↓). Instead of observing the microwave power absorbed in the ESR transition, we monitor the recombination heat deposited by ESR-induced spin-up atoms (H↑) onto a liquid-helium coated carbon bolometer. The signal from this sensor reproduces well the ESR absorption lineshape registered by our 128 GHz homodyne spectrometer. Using the recombination detection we have achieved a density detection limit of n=3·1010 atoms/cm3 for 0.2 nW microwave power incident on the resonant cavity at the temperature T=150 mK. We have studied the decay rate of recombination heat absorbed by the bolometer after an ESR excitation pulse and the dependence of this rate on T, n and nuclear polarization of the H↓ sample. The bolometer signal is found to be related mainly to second-order H↑ + H↓ recombination to ortho-H2 on the surfaces of the sample cell. From the signals we have determined the rate constant K bc c =3.2(5)·10−5√T cm2/s·K−1/2 in the interval from T=250 to 425 mK in a field of 4.5 T.

Strong magnetic compression of spin-polarized hydrogen

Fractions of spin-polarized atomic hydrogen gas samples have been compressed magnetically from a low-density buffer volume at 0.25 to 0.5 K into the pole gap of a ΔB≈6T miniature field intensifier. The density of the compressed fraction has been calculated from the observed time evolutions of the buffer volume density and nuclear polarization. It is shown that excessive recombination overheating limits the maximum density to about 2·10 19 cm -3 and thwarts the attainment of Bose condensation.

Kapitza resistance in H ↓ gas at limiting surface density

The energy exchange between a gas of spin-polarized atomic hydrogen (H ↓) and a liquid helium surface under the conditions of limiting surface density of adsorbed hydrogen atoms is discussed. It is found that the leading mechanisms of energy exchange are due to capture of gas particles into the condensate of an adsorbed H ↓ phase. The process accompanied by emission of a ripplon turns out to be effective only under a strong limitation imposed on the size of the limiting density region. The relations for the rate of energy exchange via the ripplon channel and for the Kapitza resistance at the gas-helium interface differ appreciably from those in the case of a low surface density, discussed earlier. It is shown that for T > 0.1 K the mechanism of adsorption into the surface condensate with simultaneous phonon emission in helium becomes dominant.

Compression of spin-polarized hydrogen bubbles to thermal explosion.

We have compressed spin-polarized atomic hydrogen gas in small, \ensuremath{\gtrsim}${10}^{\mathrm{\ensuremath{-}}6}$-${\mathrm{cm}}^{3}$ bubbles to densities higher than ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ using a liquid${\mathrm{\ensuremath{-}}}^{4}$He piston at temperatures from 0.3 to 0.7 K and in magnetic fields from 4.5 to 7.5 T. A best fit to the compression sweeps is obtained by fixing the binding energy of the adsorbed H atom on the liquid $^{4}\mathrm{He}$ surface to 1.15 K, which then yields the third-order dipolar recombination rate constants ${K}_{\mathrm{bbb}}^{v}$=2.7(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$ ${\mathrm{cm}}^{6}$/s for the gas phase and ${K}_{\mathrm{bbb}}^{s}$=8(2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}25}$ ${\mathrm{cm}}^{4}$/s for the adsorbed surface layer. Because of inadequate transfer of the recombination heat to the surrounding He bath, the compression sweeps terminate in a spontaneous thermal explosion of the H\ensuremath{\downarrow} bubble. The pressure at the onset of the explosive recombination event is investigated as a function of ambient temperature, bubble volume, and magnetic field. Qualitative agreement with a simple thermally activated explosion model is found.

Continuous-wave NMR in spin-polarized atomic hydrogen.

Simultaneous continuous-wave NMR and total-pressure measurements on spin-polarized atomic hydrogen gas in a 5-T magnetic field are reported. During sample loading we observe positive nuclear polarization. The rf susceptibility of the decaying sample is nonlinearly related to the density. This and the anomalous NMR line shape are discussed in light of the identical spin-rotation effect and the hydrodynamic spin-wave model. The analysis of the time evolution of the NMR signal area and of the pressure gives the two-body exchange recombination-rate coefficients ${K}_{\mathrm{aa}}$=1.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and ${K}_{\mathrm{ab}}$=1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, and the one-body relaxation rate ${G}_{1}$=1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ in the absence of rf field, at T=370 mK.

The Role of Final State Correlations in Recombination of Atomic Hydrogen

We calculate the rate-constant for recombination in the bulk of a spin-polarized atomic hydrogen gas. We use an exact initial state and include the most essential collision aspects of the final state, except for rearrangement.

Evaporative cooling of magnetically trapped and compressed spin-polarized hydrogen.

A gas of spin-polarized atomic hydrogen can be prepared in the upper hyperfine states and loaded into a static magnetic trap. Evaporative cooling and magnetic compression of such a gas can produce temperatures of 30 \ensuremath{\mu}K and densities of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Under these conditions a Bose-Einstein condensate may form.

Direct observation of a two-dimensional gas of spin-polarized atomic hydrogen.

We have used NMR to observe the two-dimensional gas of atomic hydrogen adsorbed on a fritted glass "sponge" covered with superfluid $^{4}\mathrm{He}$. Measurements were made at a magnetic field of 7 T and at temperatures down to 0.080 K for a $^{4}\mathrm{He}$ surface and to 0.060 K for a $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ substrate. The temperature dependence of the resonant frequency agrees with Anderson's multiple-residency model. On the $^{4}\mathrm{He}$ surface the adsorbed hydrogen is first detected at 0.095 K and has a frequency shift of -21.64(40) kHz with respect to the bulk adsorption line, corresponding to a zero-field wall shift of -43.0(8) kHz.

Explosive recombination of compressed spin-polarized hydrogen.

Spin-polarized atomic hydrogen gas has been compressed to densities > ${10}^{18}$/${\mathrm{cm}}^{3}$ with liquid $^{4}\mathrm{He}$ at temperatures from 0.24 to 0.67 K in magnetic fields up to 7.5 T. We find that the critical pressure, at which recombination ultimately becomes a thermally triggered explosion, goes through a maximum as a function of temperature. The explosion pressure increases with decreasing sample size and with increasing field and nuclear polarization.

Spin waves in H↓adsorbed on a superfluidHe4film

The possibilities for observation and the properties are discussed for two-dimensional spin waves in a nondegenerate spin-polarized atomic hydrogen gas adsorbed on a superfluid helium film. We present results for the spin-transport parameters D0 and µ, based on two-dimensional effective-range theory. The spin-wave quality factor is an order of magnitude smaller than in the volume case.

Magnetic confinement of a neutral gas

A three-dimensional dynamic trap for confining a collisional neutral gas is described. The trap uses the interaction of the magnetic moments of the gas atoms with a time-dependent magnetic field. Collisions lead to the evaporative cooling of the trapped atoms and to the viscous heating of the gas, particularly at high densities. This magnetic trap may allow the attainment of Bose–Einstein condensation in spin-polarized atomic hydrogen gas.

Spin waves in spin-polarized H ↓

This is a short review of previous published and unpublished results of our Cornell University group pertaining to the observations of damped nuclear spin-wave modes in spin-polarized atomic hydrogen gas.

Observation of Nuclear Spin Waves in Spin-Polarized Atomic Hydrogen Gas

Observation of Nuclear Spin Waves in Spin-Polarized Atomic Hydrogen Gas

Observation of Nuclear Spin Waves in Spin-Polarized Atomic Hydrogen Gas

We have observed narrow, distinct resonances in the NMR spectrum of dilute spinpolarized atomic hydrogen gas ($n\ensuremath{\sim}{10}^{16}$ atoms/${\mathrm{cm}}^{3}$). The dependence of the observed spectra on temperature, density, polarization, and magnetic field gradient is consistent with theoretical predictions for spin-wave excitations damped by diffusion. We have measured the parameter $\ensuremath{\mu}$, which is a measure of the importance of exchange effects in spin-transport processes, and the diffusion coefficient ${D}_{0}$, both of which are in reasonable agreement with theory.

The problem of “thermal explosion” in spin-polarized atomic hydrogen gas

The exponential character of the depolarization-rate temperature dependence in spin-polarized atomic hydrogen gas results in a “thermal explosion” phenomenon with increasing density. The inhomogeneous heating is stimulated by the three-body recombination, which in allowance for dipole-dipole interaction takes place at arbitrary spin states of colliding particles. The critical gas density is found to be in the range of 10 18–10 19 cm −3 for the real parameters of the system.